There are a number of reports of people getting warranty for sram xx world cup magnesium brakes for aluminum versions. From the reports I've read the brake fluid caused a reaction with magnesium (possibly while heated), some of the problems were bad lever feel and the need for constant re-bleeding. Do a quick search on XX magnesium brake problems and you'll find a trend.

There's a good possibility the same issues won't happen with mineral oil and shimano's design, they do seem to test their brakes far better than avid. I'll stick with 985/988 and one piece calipers until these are a bit more proven in the real world. I like the idea of shaving weight, I'm just unsure of the material.

DOT brake fluid is in-compatible with magnesium. This causes the magnesium to become porous, allowing air into the system. They discontinued the magnesium master cylinders and now they are only made in aluminum.

A co-worker had the same issue happen 2 times. The last set of XX's have aluminum master cylinders.

any more info on these brakes? Will using third party rotor make a noticeable difference in braking performance?

No going to make a difference just because they are third party, but it is important obviously to have quality rotors, not a $10 special from the bargain bin. I've ridden cheap back-up rotors on good calipers, and immediately noticed a difference in stopping power.

I'm always looking for new people to ride with. If you are on the Front Range, shoot me a PM and let's go ridin'.

I spoke with the Southeastern Shimano rep about the m987 brake. I also got to ride the new di2/hydro brake system. (freaking sweet, but a different thread altogether)

The m987 brake is made of magnesium and is a ton lighter than the m985.
Supposedly its all fine and dandy, and works just like you would expect a shimano brake to, but because of the magnesium caliper, finned pads and ice tech rotors are required.
That kind of turned me off the brake, as it almost makes up for the weight savings...
It also increases operating costs. Anyone who has recently purchased a set of finned pads or icetech rotors knows exactly what I am talking about.

I am running the m987 with the finned pads and icetech rotors and so far love the setup. I have only had them a couple weeks with ~60 miles of trail riding here in So Cal so they have seen plenty of stress stopping my 195lb ass.